The Powell River, located in southwestern Virginia and northeastern Tennessee, is a tributary of the Clinch River in the headwaters of the Tennessee River system. Historically, the Powell River had a diverse freshwater mussel fauna of 46 species. Various surveys conducted over the past century have recorded a decline in mussel densities and diversity throughout much of the river, due to historical and on-going anthropogenic impacts. In 2008 and 2009, random timed-search, systematic search, and quadrat sampling of 21 sites were completed to document species richness, relative abundance, density, and size-class structure of resident mussel populations. During the random timed search (10 sites) and systematic search (10 additional sites) portions of the survey (n=1,399 person-h), surveyors collected 15,084 mussels of 29 species. Catch-per-unit-effort ranged from 0.33 to 22.12 mussels/person-h. We observed living individuals (n = 412) of 9 of the 17 federally endangered species previously reported in the river (Dromus dromas, Epioblasma brevidens, E. triquetra, Fusconaia cor, Lemiox rimosus, Plethobasus cyphyus, Quadrula cylindrica strigillata, Q. intermedia, and Q. sparsa) and two candidate species for federal protection (Pleuronaia dolabelloides and Ptychobranchus subtentum). We recorded 19 species from 18 sites, including 5 endangered species during quadrat sampling efforts. Mean densities ranged from 0.00 to 2.25 mussels/m2 among sites sampled. Relatively recent recruitment was also evident for 16 of 29 species; including 4 endangered species (D. dromas, E. brevidens, Q. intermedia, and Q. sparsa). The mussel fauna of the lower Powell River continues to represent one of the most diverse in the United States. Outside of the Powell River, only 2 or 3 populations remain for most of the listed species extant in the river. Given these qualities, the Powell River deserves recognition as a location for focused conservation efforts to protect its diverse mussel assemblage.
INTRODUCTION
The freshwater mussel fauna of the Powell River is one of the most diverse in the United States. Historically, the river was inhabited by 46 species of mussels (Table 1). Various factors account for this diversity, such as the river valley not being glaciated during the Pleistocene epoch, a carbonate-rich lithology draining the Valley and Ridge Physiographic Province, diverse and favorable habitat types, and low level of development.
Several researchers over the past century, beginning with Ortmann (1918), have sampled mussels in much of the river (Ahlstedt, 1986, 1991a; Ahlstedt & Brown, 1979; Dennis, 1981; Ahlstedt & Jenkinson, 1987; Jenkinson & Ahlstedt, 1988; Hubbs et al., 1991; Wolcott & Neves, 1994; Ahlstedt & Tuberville, 1997; Eckert et al., 2004; Ahlstedt et al., 2005). Most recently, Ahlstedt et al. (2005) documented 36 extant species in the drainage from samples taken over a 30 y period. They reported 13 of the 17 species known from the drainage that are listed under the Endangered Species Act.
The river's mussel fauna was already experiencing a noticeable decline from anthropogenic impacts reported by Ortmann (1918). Ortmann noted that a large portion of the mussel fauna in the upper river had already been decimated downstream of a wood extraction plant located in Big Stone Gap, Virginia. Between the 1960s and 1990s, mussels in other portions of the river became increasingly rare (Dennis, 1981; Ahlstedt & Jenkinson, 1987; Jenkinson & Ahlstedt, 1988; Hubbs et al., 1991; Wolcott & Neves, 1994; Ahlstedt & Tuberville, 1997; Eckert et al., 2004; Ahlstedt et al., 2005). According to Ahlstedt et al. (2005), D. H. Stansbery used sampling data collected between 1963 and 1971 to confirm his initial observations that the mussel fauna had declined substantially in the half century since Ortmann's collections. Subsequent sampling has confirmed this decline (e.g., Wolcott & Neves, 1994; Ahlstedt et al., 2005).
Mussel declines in the Powell River have largely been attributed to habitat degradation caused by agricultural practices, urban development, and coal mining (Dennis, 1981; Ahlstedt & Tuberville, 1997; Diamond et al., 2002; Ahlstedt et al. 2005). Ahlstedt et al. (2005) considered mussel distributions and abundances to be heavily influenced by the location of mined lands in the watershed. Additional studies have shown that runoff of sediments contaminated with by-products from coal mining activities is a potential factor leading to mussel declines (McCann & Neves, 1992). Black-water events (coal fines released into the river from processing activities) have occurred frequently over the last 100 y in this watershed (Ahlstedt et al., 2005). Following a period in the early 1980s, when the entire river was known to occasionally run black with coal fines (Ahlstedt, 1986), a mussel die-off was observed in 1983 between Powell River kilometer (PRKM) 230.9 and 104.8 (Ahlstedt & Jenkinson, 1987; Jenkinson & Ahlstedt, 1988). In order to understand the effects these anthropogenic events have had on the river's diverse mussel fauna, we collected current data on species presence and abundances, distribution, and size-class structure. To that end, we utilized three different sampling techniques to assess demography, distribution, and abundance of freshwater mussels at 21 sites in the Powell River. We are providing this information so that future conservation efforts can better protect the threatened mussel fauna in the Powell River.
METHODS
Study Area
The Powell River originates near Norton in Wise County, Virginia, flows in a southwesterly direction, and enters Norris Reservoir, an impoundment of the Clinch River [at CRKM 127] (Fig. 1). The watershed drains an area of approximately 2,453 km2, and is wholly contained within the Valley and Ridge Physiographic Province. Numerous parallel ridges and subterranean drainages define the Powell River watershed (Tennessee Department of Environment and Conservation 2007). Prominent land cover includes forest (58.7%), agricultural lands (29.8%), developed, mined and barren lands (9.7%), and open water and wetlands (1.8%) (2006 NASA Landsat Data Collection [U.S. Geological Survey 2011], which were extracted via ArcMap version 9.2 using USGS 8-digit HUC [Steeves & Nebert 1994]). The mainstem of the river is characterized by long pools interrupted by periodic shallow shoals where substrate consists predominantly of a heterogeneous mix of sand, gravel and cobble.
Twenty-one sites were selected for sampling (Table 2; Fig. 1). Eighteen sites were selected based on the locations of previously documented living mussel assemblages (Dennis, 1981; Ahlstedt, 1991a; Wolcott & Neves, 1994; Ahlstedt et al., 2005; Eckert et al., 2004). Three additional sites, previously un-surveyed, were also selected because they contained accessible reaches that met the following criteria: (1) suitable shoal habitat present, and (2) where results of cursory visual and tactile survey (using snorkel gear) showed that mussels were present. We conducted these surveys in suitable habitat, which we defined as riffles and runs consisting of a stable heterogeneous mix of sand, gravel, and cobble.
Survey Approach
We employed three different survey strategies to obtain species richness, relative abundance, density estimates, and evidence of recruitment. To quantify species richness and relative abundance, one of two survey strategies was used. Based on previously obtained data (Ahlstedt et al., 2005; Wolcott & Neves, 1994; J.W. Jones, USFWS, unpublished data), if federally listed species were not likely to occur at a site, a random timed search (RTS; defined below) was used to maximize search area with minimal search time. Conversely, if federally listed species were likely to occur at the site, a systematic search (SS; defined below) was used to maximize detection. Quadrat sampling was performed at all sites to quantify density estimates and increase the probability of detecting recent recruitment.
FIGURE 1
Sites surveyed using random timed search, systematic search, and quadrat sampling methods in the Powell River.
For all survey methods, we utilized visual and tactile search methods with mask and snorkel to collect mussels. A core crew of three surveyors conducted all types of surveys; however, additional assistance was required at several sites and varied up to 20 people.
Random Timed Searches—We performed RTS at a total of 10 sites (Table 2). Surveyors initiated sampling at the downstream boundary using a series of lateral sweeps to cover as much habitat as possible within the entire delineated site, typically between 100 and 200 m. This method enabled surveyors to maximize search area while minimizing search time.
During RTS, we only collected mussels that were visible at the substrate surface and did not excavate to search for mussels. Surveyors attempted to sample the entire wetted-width of the river at each site. As mussels were found, surveyors left them undisturbed in the substrate, and marked their location with a wired florescent flag. A separate crew followed the snorkelers to remove flagged mussels and record data. Collected mussels were counted, identified to species and/or sub-species level, measured for maximum shell length (mm, anterior to posterior margin), sexed (if possible), and returned to their locations of collection. Catch-perunit-effort (CPUE) was calculated as total number of mussels divided by the amount of time spent surveying per person, expressed hereafter as person-hours (p-h). For medium to large sized (e.g., 70 – 140 mm) mussels, we assumed mussels < 40 mm in shell length were approximately 2 to 4 y old, and that the presence of mussels below this threshold showed recent recruitment (e.g., Ahlstedt et al., 2005). For smaller species (e.g. < 70 mm), we assumed mussels < 30 mm were evidence of recent recruitment.
Systematic Searches—We conducted a SS at 10 sites (Table 2) based on likely occurrences of federally listed mussels. An eleventh site, PRKM 136.2, met the criteria for this mode of sampling; however, scheduling conflicts prevented a “SS” from being conducted at this site.
For each site, we partitioned the entire shoal into 1.5 m-wide by 50 m-long sampling lanes oriented parallel to water flow using twisted masonry nylon twine stretched between two rebar stakes (1.2 m long x 1.25 cm diameter) that were pounded into the stream bottom with a drilling hammer. The number of lanes used during sampling corresponded to the width and length of suitable habitat within the river reach. A surveyor was assigned to each lane and visually searched the substrate surface of the entire area within each lane in an upstream direction. Similar to the RTS method, surveyors minimized displacement of substrate material. Mussels were marked with flags and processed as previously described under the RTS survey technique.
Quadrat Sampling—To obtain density estimates of the mussel aggregations and to determine the occurrence of recent recruitment, we excavated multiple defined quadrat areas using a systematic sampling design that incorporated a single random start adapted from Strayer and Smith (2003). We established transects that were perpendicular to flow, and were evenly spaced across the full length of each survey site. Following the selection of a starting point from the random number table, approximately ten 0.25 m2 quadrats were placed at evenly spaced intervals (2 to 5 m) along each transect. Quadrats were placed along transects in alternating directions; i.e., placed from right ascending bank to left ascending bank on first transect, followed by left ascending bank to right ascending bank on second transect. If insufficient space existed between the final quadrat on a transect and the riverbank, the difference between the remaining distance, and distance to the riverbank would be continued on the following transect, and quadrat sampling would resume. For example, if quadrats were evenly spaced at 5 m apart, and only 3 m remained between the final quadrat and the riverbank, the first quadrat on the following transect would be placed 2 m from the riverbank.
One hundred to 200 quadrats were sampled at each site. The number of quadrats sampled at each site was primarily dictated by available resources, including time and personnel. Generally, more quadrats were taken at sites deemed to have a greater likelihood of federally endangered species and allowed us to more intensively focus our quantitative effort on areas that were most important for imperiled species within the river.
Quadrat samples were taken by placing a 0.5 m x 0.5 m square constructed of 1.25 cm diameter rebar over the area to be sampled. The area within the quadrat was then excavated by hand and visually examined to a depth of 15 cm or until hardpan (a compacted layer of substrate that could not be excavated by hand) or bedrock was reached. In each quadrat, all mussels were collected, identified, sexed, measured, and denoted as visible on the surface of the substrate or undetectable at the surface. Mussels were then returned to the substrate directly adjacent to the quadrat, and substrate that was excavated from the quadrat was returned. During the quadrat survey, mussels with any portion of their shell above the substrate were noted as “exposed”, and muss∈ 
view during excavation were noted as “buried”. Quadrat data were used to estimate mussel density for each site. The precision of each density estimate was calculated post hoc using the formula: , where, n = number of quadrats sampled, m = mean number of mussels per quadrat, and CV = precision (Strayer & Smith, 2003).
Data Analysis
All summary statistics of mussel lengths and total mussel densities were calculated using Minitab 16 (Minitab, Inc., State College, Pennsylvania). Simple linear regression of total mussel densities and PRKM also was performed using Minitab 16. P-values < 0.05 were considered significant. Tables and figures were created using Excel 2007 (Microsoft, Inc., Redmond, Washington). The site map was produced using ArcMap 9.2 (Environmental Systems Resource Institute (ESRI), Redlands, California).
RESULTS
Mussel Surveys
Based on RTS and SS, a total of 15,084 mussels representing 29 species were collected among the 21 sites surveyed (Tables 3 and 4). Species richness ranged from 1 to 23 species per site (x±SE; 13.8 ± 1.58), with the highest number at PRKM 152.6 and the fewest at PRKM 263.0 (Table 3; Fig. 2). Total live mussels ranged from 1 (PRKM 263.0) to 4,297 (PRKM 152.6) mussels (754 ± 240) per site (Table 3). Total CPUE ranged from 0.33 (PRKM 263.0) to 22.12 (PRKM 152.6) mussels/p-h (8.68 ± 1.68; Table 3). Evidence of relatively recent recruitment was observed for 16 of 29 species collected live among nine sites (Table 4). Of the 15,084 mussels collected during RTS and SS sampling, 74 (0.5%) were considered recent recruits (Table 4).
Quadrat sampling (n = 2,580) yielded 580 mussels of 19 species from 18 of 21 sites (Table 5). Mean densities ranged from 0.00 (PRKMs 269.4, 266.3, and 263.0.) to 2.25 (PRKM 135.8) mussels/m2 among sites (0.88 ± 0.144) (Table 6). A significant linear relationship was shown between mussel density and PRKM (r2 = 0.295, F = 7.94, df = 20, P = 0.011; Fig. 3). Similarly, a significant relationship occurred between PRKM and the number of species collected during quadrat sampling (r2 = 0.655, F = 36.10, df = 17, P < 0.001; Fig. 2). Precision of density estimates ranged from 0.09 to 0.22. Species richness among sites ranged from 0 (PRKMs 269.4, 266.3, and 263.0) to 10 (PRKM 180.7) species (5.81 ± 0.75).
FIGURE 2
Species observed at selected sites in the Powell River during random timed search, systematic search, and 0.25 m2 quadrat sampling. Systematic sampling was not conducted at PRKM 136.2 due to resource constraints. Dark bars: Number of species collected during random timed search and systematic search; Light bars: Number of species collected during quadrat sampling. Statistically significant linear relationship between number of species collected during quadrat sampling and PRKM: r2 = 0.655, F = 36.10, df = 17, P < 0.001; * = site where propagated juveniles have been released.
Of 580 mussels collected from quadrats, 21 (3.6%) individuals were deemed to be relatively recent recruits among six species (A. pectorosa, E. dilatata, E. brevidens, L. ovata, M. conradicus, and V. iris) over nine sites (Table 5). For species that were sexually dimorphic, all but two species (L. ovata and V. iris) were represented by both male and female individuals.
DISCUSSION
The results of this survey show that a speciose mussel fauna still inhabits the lower Powell River, including at least 11 federally endangered and candidate species. For example, the presence of relatively recent recruits of the critically imperiled Quadrula intermedia and Quadrula sparsa illustrates the importance of continued conservation efforts in the basin. However, despite the presence of diverse, recently recruiting populations, the fauna has likely lost one-third of its species since Ortmann (1918) (from 46 species historically to 29 current species) (Table 1).
Although not collected during this survey, Cumberlandia monodonta, Fusconaia cuneolus and Pleurobema oviforme could still inhabit the river at undetectable levels. While live individuals of C. monodonta were not collected, fresh-dead specimens indicated that the species probably persist in the Powell River. Both F. cuneolus and P. oviforme may also inhabit the river in very low densities, because both having been collected in recent decades (Eckert et al., 2004). In addition, only a few older individuals of Potamilus alatus and Q. pustulosa were collected. However, P. alatus is probably more common than our sampling indicated, because slackwater, soft substrate habitat was not adequately surveyed using our site selection criteria targeting shoals.
FIGURE 3
Estimated mussel densities at selected sites in the Powell River, utilizing 0.25 m2 quadrat sampling. Statistically significant linear relationship between estimated mussel density and PRKM: r2 = 0.295, F = 7.94, df = 20, P = 0.011; * = site where propagated juveniles have been released.
Neither live individuals nor shell material of Epioblasma capsaeformis or Hemistena lata were collected during this survey. The last evidence of E. capsaeformis comes from the collection of a single individual at PRKM 193.4 in the late 1980s (Wolcott & Neves, 1994). The last evidence of H. lata was a single shell collected from PRKM 179.9 in the late 1990′s by J. Jones (unpublished data). Given the short life spans of these species (< 15 y) (Watters et al., 2009; Jones & Neves, 2011), any remnant individuals have likely been extirpated from the river. However, H. lata may still reside in the river because it is difficult to detect (individuals burrow deeply (10 to 15 cm) in the substrate (Ahlstedt, 1991b), and old shell material degrades quickly. Alasmidonta marginata, A. viridis, Pegias fabula, Strophitus undulatus, Toxolasma lividum, Truncilla truncata, Villosa fabalis, and Villosa perpurpurea have not been collected in the past several decades and are very likely extirpated from the river. These species are considered headwater forms and have likely been impacted by upstream pollution (Ahlstedt & Brown, 1979; Dennis, 1981). Epioblasma torulosa gubernaculum, once documented as inhabiting the Powell River, is believed extinct (Williams et al., 1993). Although Lasmigona holstonia has been seemingly extirpated from the mainstem of the Powell River, the species is still extant in Beaverdam Creek, a tributary of the South Fork Powell River, upstream of Big Stone Gap, Virginia (The Catena Group, 2008).
As documented in previous surveys (Ahlstedt & Brown, 1979; Dennis, 1981; Wolcott & Neves, 1994), a significant decline in both species diversity and mussel abundance was observed, particularly in an upstream direction and above the island at Snodgrass Ford. This decline has been attributed primarily to coal mining, but also to agriculture, and effects from urban areas have been implicated (Ahlstedt & Tuberville, 1997; Wolcott & Neves, 1994; Ahlstedt et al., 2005). These activities will likely continue in the upper Powell River watershed over time. In addition, natural gas extraction is expanding throughout the Appalachian region and may become a factor in the future (Zoback et al., 2010; Osborn et al., 2011). During our survey, large amounts of sediment were evident in both the water column and covering the substrate surface at the most upstream sites (e.g., above PRKM 130.9). It has been suggested that sedimentation can lead to reduced reproductive success in some mussel species (Brim Box & Mossa, 1999). It is generally believed that the major decline in mussels of the Powell River headwaters is attributable to coal mining activities, and associated contaminants (e.g., McCann & Neves, 1992). The role of stressors on the mussel fauna, particularly in the upstream portions of the Powell River (upstream of PRKM 206.6), needs further study to determine their effects on all life-history stages.
Dam construction in the upper Tennessee River system will continue to have a legacy effect on Powell River mussels. Low abundance and large size indicates a long-term lack of recruitment for Elliptio crassidens, Ligumia recta, and Truncilla truncata (Table 4). This may be caused by a virtual loss of their primary host fishes, skipjack herring (Alosa chrysochloris) and sauger (Sander canadense). This loss of host fish is due to downstream dams blocking their spawning runs. As a result, extirpations of species like Elliptio crassidens and Ligumia recta can be expected due to the extinction debt caused by habitat fragmentation (Tilman et al., 1994).
Evidence of recent recruitment is an indicator of population viability. It is important to note that sub-adults of multiple species were collected during this survey, albeit in low numbers (3.1% of total abundance in quadrats), including several endangered species (Epioblasma brevidens, Lemiox rimosus, Pleuronaia dolabelloides, Q. intermedia, and Q. sparsa). Nonetheless, this is evidence that portions of the lower Powell River continue to support recruiting populations of federally endangered species and that the differences between these reaches and reaches that do not support recruitment should be studied further.
Due to the presence of several recruiting federally endangered species (e.g., E. brevidens, Q. intermedia, and Q. sparsa), the section between PRKMs 153.4 and 152.6 is perhaps the most productive reach in the river. Based on our search methods, 7 of the 8 endangered species found at PRKM 152.6 had their greatest abundance at that site (161 individuals), representing 39.1% of the total. The greatest abundance (28.5% of total abundance) and the highest CPUE (22.1 mussels/p-h) were also found at this site. This is significant because despite the presence of the Quadrula species in other sections of the river, young individuals were not collected outside of this reach. In addition, only one other recruiting population of Q. intermedia is known to exist (Duck River of central Tennessee), and no additional recruiting populations of Q. sparsa are known to occur elsewhere (Parmalee & Bogan, 1998). For these reasons, it is important that this reach of the river be protected.
In addition to the river section between PRKMs 153.4 and 152.6, the river section between PRKMs 197.9 and 188.8 is also of particular interest for future conservation efforts. The sites at PRKMs 193.4 and 188.8 have been release sites for propagated juveniles of both common and threatened species (Eckert et al., 2004). Densities at these sites were among the highest of sites sampled, which may be in part due to these juvenile releases. The mussel densities at PRKM 197.9 are also among the highest of the sites sampled during this study. The mussel aggregations at this site have not been frequently sampled like some adjacent sites (Eckert et al., 2004; Ahlstedt et al., 2006), so declines at this site have not been as thoroughly monitored. Additional sampling should be conducted near this site to determine why densities at this site have not declined to the extent that they have both upstream and downstream of this reach at un-augmented sites.
The mussel fauna of the Powell River continues to be threatened by numerous anthropogenic activities. Despite these impacts, the river still contains one of the most diverse mussel faunas in the United States. Among national rivers, only the Clinch River harbors more extant populations of naturally occurring federally endangered mussels. Although low, there was evidence of recruitment at a number of our sample sites, indicating that the Powell River, if managed correctly, has the potential to rebound from ongoing and historical anthropogenic impacts. It is imperative that research, habitat and population restoration, and monitoring efforts continue in this river to conserve its speciose mussel fauna.
TABLE 1
Conservation status for mussel species known from the Powell River. American Fisheries Society (AFS) status from Williams et al. (1993). Conservation Status: CS = currently stable, E = endangered, FE = federal endangered, FC = federal candidate, PE = federal proposed endangered, T = threatened, V = vulnerable or special concern, X = possibly extinct, - = no status and √ = considered extant based on current study.
TABLE 2
Site locations, site numbers, site names and survey methods used in the Powell River
TABLE 3
Numbers and relative abundances of each species collected during random timed search and systematic search at selected sites in the Powell River. Total numbers of mussels collected, catch-per-unit-effort (CPUE), and total species collected are also provided for each site.
Continued
TABLE 4
Summary statistics of abundance, length, and recruitment for mussel species collected in the Powell River, during random timed search and systematic search. F = female, M = Male, U = Sex Undetermined.
TABLE 5
Summary statistics of abundance, length, and recruitment for mussel species collected in the Powell River, during 0.25 m2 quadrat sampling. S = # of mussels found on substrate surface, B = # of mussels found buried in substrate, F = female, M = Male, U = Sex Undetermined.
TABLE 6
Estimated densities (mussels/m2) of each species at each site during 0.25 m2 quadrat sampling in the Powell River. Total density estimates and standard errors (SE), density estimate precision, and total species collected are also provided for each site. * = site where propagated juveniles have been released.
ACKNOWLEDGEMENTS
We thank the U.S. Fish and Wildlife Service for providing funding to complete this project. Charles Randklev and two anonymous reviewers provided comments that improved the quality of this manuscript. We are also thankful to S. Ahlstedt, B. Beaty, C. Carey, J. Carter, D. Hua, M. King, N. King, T. Lane, B.J.K. Ostby, B. Watson, and A. Wilson for their assistance in conducting surveys.
